1. Field of the Invention
The present invention relates to a digital/analog converter converting a digital signal of a binary number into an analog signal.
2. Prior Art
As an applied-voltage digital/analog converter, that which employs an R-2R system is conventionally known. FIG. 11 shows an 8- bit digital/analog converter employing such an R-2R system.
As shown in FIG. 11, the digital/analog converter is provided with eight changeover switches S.sub.0 to S.sub.7 corresponding to a bit in each order of an 8-bit digital signal to be D/A-converted. One of contact points of each of the changeover switches S.sub.0 to S.sub.7 is connected with a first input terminal 1 to which reference voltage V.sub.REF is applied, while the other is connected to a second input terminal 2 to which ground potential GND is applied. Between the second input terminal 2 and an output terminal 3, a single resistor which has a resistance value 2R and seven resistors each of which has a resistance value R are connected in series. Between respective nodes between the above-mentioned resistors and respective common contact points of the changeover switches S.sub.0 to S.sub.7, eight resistors each of which has a resistance value 2R are connected, respectively.
With the digital/analog converter, the changeover switches S.sub.0 to S.sub.7 change their respective connections between the respective first contact points to which the reference voltage V.sub.REF is applied and the respective second contact points to which the ground potential GND is applied depending upon a state of a corresponding bit of a digital signal, and an analog signal having a voltage value corresponding to the digital signal is outputted from the output terminal 3.
In this way, the digital/analog converter employing the conventional R-2R system do not have to have a large number of elements such as the resistors, the changeover switches S.sub.0 to S.sub.7 and so on, and thus this type of digital/analog converter has the advantage that it can be controlled in a simple way. However, it also has the disadvantage as mentioned below: Practically, the resistors have slightly different resistance values from their desirable resistance values R or 2R, and an error arises in a variation in an output of the analog signal corresponding to a variation in a digital data. With the conventional digital/analog converter, an error included in the variation in an output of an analog signal which is caused by the variation in the digital data for the least significant bit exerts a large influence on the performance of the digital/analog converter, and therefore, it is necessary to accomplish a very high matching of resistors to obtain a converter which has a high resolution, keeping a monotonicity that the analog value of the analog signal monotonously increase or decrease in accordance with the monotonous change of the digital value of the digital signal.